A number of nucleoside analogs such as cytarabine, fludarabine, cladribine, capecitabine, gemcitabine, and pentostatin are used clinically as highly effective anti-neoplastic agents. Among these, gemcitabine (2′,2′-difluoro-2′-deoxycytidine, Gemzar™) is of particular interest due to its unique activity against solid tumors and is presently used therapeutically to treat bladder, breast, lung, ovarian and pancreatic cancer.
Several self-potentiating mechanisms unique to this nucleoside analog are believed responsible for the activity of gemcitabine against solid tumors. The diphosphate metabolite of gemcitabine inhibits ribonucleotide reductase, which results in lower concentrations of intracellular deoxycytidine triphosphate (dCTP) and thus increased incorporation of the triphosphate gemcitabine metabolite into DNA, which results in inhibition of DNA synthesis and blocks completion of the cell division cycle. Additionally, reduction in dCTP concentration upregulates the enzyme cytidine kinase, which is responsible for the initial phosphorylation of gemcitabine, a necessary step in the inhibition of DNA synthesis by gemcitabine. Finally, the triphosphate metabolite of gemcitabine is an inhibitor of cytidine deaminase, which is responsible for gemcitabine inactivation by conversion to the uridine metabolite. Accordingly, the additive nature of the above factors may explain the efficacy of gemcitabine in treating solid tumors. Synthetic derivatives of gemcitabine, including several prodrug compounds, have been previously described (see for example, Ishitsuka et al, International Publication No. WO03/043631; Alexander et al, J. Med. Chem. 2003, 46, 4205-4208; Greenwald et al, U.S. Pat. No. 6,303,569; Guo et al, Cancer Chemother. Pharmacol. 2001, 48, 169-176; Greenwald and Choe, International Publication No. WO01/21135; Di Stefano et al, Biochem. Pharmacol. 1999, 57, 793-799; Guo et al, J. Org. Chem. 1999, 64, 8319-8322; Greenwald et al, International Publication No. WO99/33483; Myhren et al, International Publication No. WO98/32762; Zhao et al. International Publication No. WO98/00173; Chou et al, U.S. Pat. No. 5,606,048; Chou, U.S. Pat. No. 5,594,124; Chou et al, European Patent Application No. EP712860; Wildfeuer, U.S. Pat. No. 5,521,294; Kjell, U.S. Pat. No. 5,426,183; Chou, U.S. Pat. No. 5,401,838; Bonjouklian et al, European Patent No. EP0376518; Chou et al, European Patent Application No. EP577303; Hertel et al, European Patent Application No. EP576230; Chou et al, Synthesis 1992, 565-570; Richardson et al, Nucleic Acid Res. 1992,20, 1763-1769; Baker et al, J. Med. Chem. 1991, 34, 1879-1884; Klaveness and Undheim, International Publication No. WO91/15498; Hertel et al, European Patent Application No. EP329348; Koppel et al, European Patent Application No. EP272891).
Previous studies have characterized multiple cellular transport mechanisms for nucleoside analog drugs and their derivatives (for a review, see Balimane and Sinko, Adv. Drug Delivery Rev. 1999, 39, 183-209). A relatively hydrophilic compound, gemcitabine has limited ability to permeate plasma membranes via passive diffusion, and several studies have demonstrated that gemcitabine is a substrate for equilibrative and concentrative nucleoside transporters (ENT's and CNT's respectively). Specifically, gemcitabine is transported by human ENT1, ENT2, CNT1 and CNT3, but not the purine-selective concentrative transporter CNT2 (see Mackey et al, Cancer Res. 1998, 58, 4349-4357; Mackey et al, J. Natl. Cancer Inst. 1999, 91, 1876-1881; Fang et al, Biochem. J. 1996, 317, 457-465).
The superior efficacy of gemcitabine against solid tumors makes gemcitabine a particularly compelling candidate for conversion to an orally bioavailable prodrug. Oral administration of cancer drugs may provide for a more convenient dosing regimen and greater efficacy than intravenous infusions of cancer drugs, which are typically used in cancer therapy. Intravenous infusions of cancer drugs require long intervals between administration to allow the patient to recover from the treatment as the IV doses given are near their toxicity limit. Oral delivery of cancer drugs may allow for the attainment of a much more attractive plasma concentration profile in which the concentration of the drug is kept just above its therapeutic threshold for an extended period. Consequently, the patient may experience fewer side effects, while still obtaining the anti-neoplastic benefit of the drug.
However, a significant problem with oral administration of gemcitabine is the intestinal toxicity of this nucleoside analog. Accordingly, there is a need for gemcitabine analogs, which lack intestinal toxicity while retaining the efficacious anti-tumor activity of the parent drug.